This invention relates to spacecraft fabricated from panels, and more particularly to fasteners and fastening methods which reduce the weight and cost of panel assembly.
The basic structural members of spacecraft currently include support panels in the form of relatively high-strength face sheets, spaced apart by a low-density core. As an example, such a panel might include a pair of parallel, spaced-apart aluminum sheets, with an aluminum honeycomb bonded therebetween. Such a panel exhibits high strength for its weight, and has advantageous thermal and electrical conduction characteristics. Other materials may be desirable, as for example the face sheets may be made from carbon-fiber reinforced resin.
Electrical and mechanical components and subassemblies are mounted on various panels of the spacecraft. Because of the high reliability required of a spacecraft, many tests are performed on the individual components, and on the subassemblies, at various stages of the manufacture. As part of assembly of a spacecraft, it is often necessary to fasten such a panel to another panel, either at the edges or at locations away from an edge. It might be possible to adhesively bond together the various panels of the spacecraft. However, adhesively bonded panels could not be disassembled for test or for access to other components for test. Consequently, removable fasteners are ordinarily used for holding together spacecraft panels,
FIG. 1a illustrates a pair of panels 2, 4, and a portion of the housing 5 of a piece of electrical equipment mounted on panel 2, together with a prior art fastening technique. In FIG. 1a, panel 2 is formed from face sheets 6a and 6b, separated by a low-density core of honeycomb material 6c. Panel 4 includes face sheets 8a, 8b, and a similar honeycomb core 8c. Panels 2 and 4 are fastened together by a plurality of fastening arrangements 10.sup.1, 10.sup.2, 10.sup.3, 10.sup.4. . . as illustrated in FIG. 1d, only one of which, designated generally as 10, is illustrated in FIG. 1a. Fastening arrangement 10 includes a first clevis 10a, a second clevis 10b, and a bolt 24. Clevis 10a is associated with panel 2, and includes a furcated portion 12', 12" and a connecting body portion Portions 12' and 12" of clevis 10a straddle portions of the face sheets of panel 2, and are adhesively fastened thereto. Clevis 10b is associated with panel 4, and includes a furcated portion 14', 14" which straddles across, and is adhesively fastened to a portion of face sheets 8a and 8b. Clevis 10b includes a flat body portion 16, which has a close-fitting clearance hole 24 for the body portion 27 of bolt 24, and which is fitted with a threaded captivated floating nut fastener 40, described below in conjunction with FIG. 1c. Body 18 of clevis 10a holds furcated portions 12', 12" in mutually parallel relationship, and supports a flat base or bracket 20 which defines an aperture 22.
Bolt 24 of FIG. 1a includes a head portion 26 and a threaded end portion 28, with a body portion 27 lying therebetween. Bolt 24 extends through aperture 22 in clevis 10b, and its threaded end 28 screws into a floating nut 46, described below is conjunction with FIG. 1c. When bolt 24 is tightened into its nut, the head 26 of bolt 24 bears against a portion of bracket 20, and the adjoining faces of bracket 20 and the body portion of clevis 10b abut along a plane 32, visible in the cross-section of FIG. 1b.
A floating nut assembly designated 40 is fastened behind aperture 24 in FIG. 1a. The nut 48 must float, so that any slight misalignment between the nut and aperture 24 may be taken up during assembly. FIG. 1c is a perspective view of floating nut assembly 40. As illustrated, floating nut assembly 40 includes a base portion with bent-over lugs 48, which captivate a one-piece nut portion including a base portion 44 and a projecting threaded portion 46. The piece represented by base 44 and threaded portion 46 is free to move slightly relative to base 42. The floating nuts have locking features (deformed threads) which prevent loosening of the bolt, but which allow for repeated disassembly and reassembly.
During assembly, the panels are clamped in their desired relative positions, which are illustrated in FIG. 1d. If the clevises have not already been placed in position, they are so placed and adhesively fastened. If holes 22 in clevis 10a and 24 in clevis 10b have been preformed in the clevises before assembly, it is likely that the holes 22, 24 in fastener pairs will not properly align, unless some cutting or adjustment is made during the clevis adhesive fastening process. When there are many such fasteners associated with a pair of panels, it may be possible to adjust one or two fasteners so that holes 22 and 24 are coaxial, but the remainder are unlikely to align exactly.
In order to avoid the cost of hand-crafting the connection of each clevis to its associated panel, aperture 22 may be predefined in bracket 20 of clevis 10a with a diameter greater than the body diameter of bolt 24. The oversize of aperture 22 must be sufficient to accommodate the maximum expected tolerance error, to avoid the necessity for a reaming operation to enlarge the aperture. Reaming is undesirable, because the panel assembly normally takes place after equipment, such as the equipment represented by housing portion 5 of FIG. 1a, has been fastened to the panels and tested. A reaming operation generates chips and debris which could lodge within electronic components, or on surfaces to which further devices are to be fastened or which are intended for relative motion, thereby creating the possibility of electrical short circuits, mechanical interference, or both.
FIG. 1b is a cross-section of the assembled joint of FIG. 1a. Elements of FIG. 1b corresponding to those of FIG. 1a are designated by like reference numerals. As illustrated in FIG. 1b, the edges of aperture 22 do not bear against the body of bolt 24. The edges of aperture 24, however, do bear against the body of bolt 24. In order to prevent relative motion between the panels at fastener arrangement 10, bolt 24 is tightened into floating nut 46 sufficiently so that, in the presence of shear forces, friction between flat body portions 16 and 20 prevents motion. In FIG. 1b, the shear forces are represented by arrows 30. A useful rule of thumb for materials of the type used in spacecraft is that the normal forces holding together fastener halves 10a and 10b at an interface surface 32 must be about three times the expected shear force. Therefore, in order to withstand a shear force of, for example, 800 pounds force (1 bf), the normal force exerted by screw 24 must be on the order of 2400 lbs. In a particular such application, it is expected that, even using high-strength titanium fasteners, 1/4-inch bolts are required so as not to exceed their tensile limits. The relatively large number of bolts of such relatively large size represents a significant weight on the spacecraft. Also, the relatively large normal forces required, as described above, mandates a thicker cross-section for bracket 20 and body 16 of clevises 10a and 10b, respectively, all of which undesirably adds weight to the spacecraft.
FIG. 2a illustrates a prior-art arrangement for connecting the edge of a panel to a location away from the edge of a second panel. Elements of FIGS. 2c and 2b corresponding to those of FIGS. 1a and 1b are designated by like reference numerals. In FIGS. 2a and 2b, clevis 10b and panel 4 are identical to those of FIGS. 1a and 1b. Panel 2, however, is fitted with a "well" fastener designated generally as 50, which is in the general form of a cup, including a projecting lip 52, a body portion 54, and a bottom portion 56 defining an oversize aperture 22. The depth of body 54 is selected so the well fastener bottom is flush with the reverse side (not visible in FIG. 2a) of panel 2 when projecting lip 52 is flush with face sheet 6a of panel 2. The correspondence of well fastener 50 to clevis 10a is clear, and its cross-section is very similar to that of FIG. 1b. The arrangement of FIG. 2a suffers from the same problems as those of the edge fastener of FIGS. 1a, 1b and 1c.
If aperture 22 could be made to closely fit about the body of bolt 24 in FIGS. 1a or 2a, shear forces could be carried in a "body bound" fashion from the edges of aperture 22 directly to the body of bolt 24, and thence directly to the edges of hole 24. With such an arrangement, shear forces would be coupled directly from one clevis to the other clevis through the body of bolt 24, and motion would be prevented without reliance upon friction at the junction. As a result, large normal forces would not be required, the tensile stresses in the bolt could be reduced, and smaller bolts could therefore be used, together with a reduction in the cross-sectional dimensions of the associated portions of the clevises. An overall weight reduction would result. Such body bound construction is known in industries such as the aircraft industries. The body bound construction is accomplished in the aircraft industry by, with the structures being clamped in their desired positions, drilling apertures, corresponding to apertures 22 and 24, simultaneously. This results in lightweight, accurate fabrication, but at the expense of "crafting" each fastener, and also at the cost of undesirably producing debris and chips at a late stage in fabrication. Improved fastening methods are desired.